editor: fix missing user/password when loading wifi connection to edit

When you opened an existing wifi in nm-connection-editor, the username
and password field were unset for WPA & WPA2 Enterprise.

(This regression was introduced by commit c9476e9c07839e71aed49e820e364ad0b9f47039)

https://bugzilla.redhat.com/show_bug.cgi?id=1000564

Signed-off-by: Thomas Haller <thaller@redhat.com>
editor: remember/restore old password when toggle ask_password in wifi

In the wifi security dialog, when you set the "always ask" checkbox, the
password will be cleared. Remember the previous password and restore it when
you uncheck the box again.

This has the effect, that you can toggle this checkbox and don't have to
reenter the password.

This only works, as long as you don't change the "Security",
"Authentication" or "Inner authentication" type. This is
intentionally not to expose the password by accident.

Signed-off-by: Thomas Haller <thaller@redhat.com>